3D view building/manipulation

I use them both. I use the 3d For the basic shape, then zoom REALLY closely in, to line up the brushes, or whatever I'm working with.

I really only work in 3D for giving me a rough idea of where to put stuff, and almost exclusively for overlays.

I only work in 2d. in 3d the risk of getting leaks or not properly aligned brushes is too great. plus i was already used in 2d working before i even knew hammer (due to red factions red editor where only 2d was possible). Only for overlays, decals and selecting brushes i use the 3d view. But its allways followed with 2d editing (even with resizing decals).

3D only 4 decals/overlays.

And what about props? I find positioning "junk" items like boxes barrels rocks bushes etc. to be much better in the 3D view because you can do it in...3D...and it doesn't need to be precise.

I mostly work in the 3D view at any rate, most brushes and dispalcements in my maps are clone dragged and rescaled copies of their neighbors, though precision lining up of detailed brush work and efffect entities like ropes, light glows, etc. is better in 2D view, I don't have any trouble with lining up world brushes in 3D so long as your grid is set high...